Prepared purified fluorescent derivatives of prolactin, insulin, and growth hormone by fluorescent labeling, followed by column chromatography or gell electrophoresis. Prepare analogous derivatives of estradiol and testosterone. Prepare by known methods clelular receptors from normal and neoplastic mammary tissue of normal rats and rats bearing DMBA mammary tumors. Later receptors may be prepared from established human breast tumor cell lines. Fractions originating from plasma membrane and endoplasmic reticulum will be treated separately from those of the cytoplasm or the nucleus. Investigate by means of equilibrium and kinetic fluorescence polarization measurements the interaction between fluorescence labeled hormones and their specific cellular receptors.